Penetration Depth, Spatial Resolution, Data Density, and Distance CoveragePenetration depth and spatial resolution for Geoscy antennas are listed in Table 1. The greater the depth to which a given antenna signal penetrates, the lower is its resolution in the vertical direction.
The exceedingly fast data collection rate of the state-of-the-art SIR 4000 controller means higher data density which increases signal recognition. Alternatively, faster GPR data collection permits GPR profiling over much larger distances than previously possible. Geoscy has the capability to profile at speeds of ~10 mph on highways using our non-metallic trailer, permitting reconnaissance GPR mapping of tens of miles per day.

Field PlanningOnce client objectives and site conditions are ascertained, several considerations go into survey design. We are inclined to use antennas at multiple frequencies because the overlap in penetration depths aids interpretation of data from each of the antennas. Given the high data-collection rate and software advances, it is very useful to collect GPR profiles along a grid of lines. Therefore, grid location, spacing of profile lines within the grid, and also orientation of the profiles are part of survey design. Whether or not a grid is appropriate, we use multiple methods to obtain geographic coordinates for the GPR lines and, potentially, topography.

Fig. 6. Three-person GPR profiling with a bistatic 100 MHz antenna.

Field Data CollectionThe first phase of field work involves experimental profiling to tune GPR controller parameters and finalize location/orientation/marking of the transects. Preliminary GPR data are analyzed real-time to locate transect lines. If subsurface samples had not already been collected and analyzed, some form of sampling should be accomplished to aid data interpretation. Then follows collection of all GPR survey data (Figs. 6-7). We collect GPS data simultaneously. The geographic coordinates of the transect lines must be precisely surveyed.